1. Technical Field
The present disclosure related to an input/output (I/O) module, and more specifically, to the I/O module configured to connect a controller, which configures a variety of control systems of a power plant, a manufacturing process, water and sewerage, urban gas and the like, and a field device provided at an outside of the control system.
2. Related Art
FIG. 2 is a block diagram for one channel, which depicts a connection example of a field device in a control system of the related art. In FIG. 2, a controller 100 and a field device 200 are connected via an interface circuit 300.
The interface circuit 300 has a signal unit 310 and an output unit 320.
The signal unit 310 has a FET (field effect transistor) 311 functioning as a switching element, a current source 312, a filter 313, and a buffer amplifier 314.
A source of the FET 311 is connected to a direct current power supply line of +24V, one end of a resistance 312b configuring the current source 312 and one end of a capacitor 313a configuring the filter 313. A gate of the FET 311 is connected to the controller 100. A drain of the FET 311 is connected to a cathode of a clamp diode 321 configuring the output unit 320 and a drain of a FET 312a configuring the current source 312.
The current source 312 has the FET 312a, the resistance 312b and an operational amplifier 312c. 
A source of the FET 312a is directly connected to one input terminal of the operational amplifier 312c and one end of a resistance 313b configuring the filter 313 and is also connected to the source of the FET 311 and the one end of the capacitor 313a configuring the filter 313 via the resistance 312b. A drain of the FET 312a is connected to the cathode of the clamp diode 321 configuring the output unit 320 and the drain of the FET 311. A gate of the FET 312a is connected to an output terminal of the operational amplifier 312c. 
The other input terminal of the operational amplifier 312c is connected to the controller 100.
The filter 313 has the capacitor 313a and the resistance 313b. 
A connection point of the capacitor 313a and the resistance 313b configuring the filter 313 is connected to the controller 100 via the buffer amplifier 314.
The output unit 320 has the clamp diode 321. An anode of the clamp diode 321 is connected to a direct current power supply line of 0V, and the cathode is connected to the drain of the FET 311 and the drain of the FET 312a. Both ends of the clamp diode 321 are connected with the field device 200.
In the above circuit configuration, the controller 100 and the interface circuit 300 are configured to operate in conjunction with each other so that the connection line L1 is involved at a digital output (DO) mode, the connection line L2 is involved at an analog output (AO) mode, and the connection line L3 is involved at an analog input (AI) mode and a digital input (DI) mode, for example.
The current source 312 is used at the analog output (AO) mode, and the FET 311 functioning as the switching element is used at the digital output (DO) mode.
Patent Document 1 discloses the circuit configuration shown in FIG. 2 and operations thereof.
[Patent Document 1] U.S. Pat. No. 8,392,626B
However, according to the connection configuration of the field device of the related art shown in FIG. 2, a signal line, which is configured to transmit a control signal, of the signal lines configured to electrically connect the controller 100 and the interface circuit 300 includes an analog signal, and a signal for each function mode is provided. Therefore, the cost of the controller 100 increases and a mount area of a circuit component increases.
Also, it is necessary to separately provide the current source 312 that is to be used at the analog output (AO) mode and the FET 311 functioning as the switching element that is to be used at the digital output (DO) mode, which also increases the cost and the mount area.
Also, the current source 312 that is to be used at the analog output (AO) mode should be provided with an operational amplifier for each channel so as to keep the output precision of each channel, which also increases the cost and the mount area.
Further, since it is necessary to provide the circuit configuration of FIG. 2 for each channel, the cost and the mount area increase in proportional to the number of channels.
As a circuit size of the interface circuit 300 increases, a size of the interface circuit 300 also increases, so that the power consumption and the heat generation also increase and the system is enlarged.
The above factors are obstacles to the miniaturization of the control system that a user desires and thus are not favorable.